Since the 1986 paper of P. Powell-Abel et al [see Science 223:738] showing that plants transformed with and expressing the coat protein gene of tobacco mosaic virus (TMV) are resistant to TMV, there have been a number of other examples of this concept which will undoubtedly have important implications for the protection of many crop species from various viral infections. To date, for example, viral coat protein-mediated resistance has been shown with at least 25 viruses in 15 taxonomic groups including alfalfa mosaic virus, tobacco rattle virus, potato virus X, cucumber mosaic virus (CMV), potyviruses, and plants transformed with both potato virus X and potato virus Y coat protein.
Plant virus sequences other than those coding for the viral coat protein have been tested to determine if transformed plants can be made to exhibit resistance to post-transformation viral infection. Positive sense sequences of alfalfa mosaic virus comprising almost full length copies of RNAs 1 and 2 failed to induce resistance in transformed plants [see Virology 163:572 (1988)]; anti-sense sequences of the TMV and potato virus coat protein genes did induce a low level of resistance in transformed tobacco [see Proc. Nat""l Acad. Sci., USA 86:6949 (1989); and EMBO Journal 7:1273 (1988)]; likewise antisense RNAs from one of three region, tested (5xe2x80x2 sequences of RNA 1) of the CMV genome gave a low level of resistance in one transformant line.
Other forms of resistance using plant transformations with DNAs prepared from satellite RNAs of plant viruses have been reported, such as the use of the satellite of CMV [see Nature 328:799 (1987)] and the concept of the ribozyme based on sequences from satellite RNAs which possess the capacity to self cleave [see Nature 334:585 (1988)].
The invention described herein represents an entirely new type of virus-induced resistance which may be transferred from one plant generation to another. The present invention discloses that transgenic plants containing a coding sequence, taken from the read-through portion of the replicase portion of the viral genome, are resistant to subsequent disease by the virus; although there may be a very benign degree of virus synthesis in the inoculated leaf, the virus has been found not to spread and hence no disease develops. In the description which follows, the use of the 54 kDa coding sequence from TMV is described as a specific example of the broader technology according to the present invention. Thus, in its broadest aspects, the present invention defines a means for bringing about viral resistance in plants which have been transformed with nucleic acid copies of fragments or segments taken from the replicase portion of the pathogenic virus genome. In addition, the present invention defines transformed plants and their seeds which carry a portion of the viral genome which codes for a portion of the read-through portion of the replicase genome of the pathogenic virus. According to the present invention, transformed plants that contain a portion of the viral replicase gene within their genome are resistant to subsequent viral disease from the virus from which the portion was derived, and these plants may also be resistant to subsequent disease from other closely related viruses.